RUP2 facilitates UVR8 redimerization via two interfaces

The plant UV-B photoreceptor UV RESISTANCE LOCUS 8(UVR8)exists as a homodimer in its inactive ground state.Upon UV-B exposure,UVR8monomerizes and interacts with a downstreamkey regulator,theCONSTITUTIVE PHOTOMORPHOGENIC 1/SUPPRESSOR OF PHYA(COP1/SPA)E3 ubiquitin ligase complex,to initiate UV-B signaling.Two WD40 proteins,REPRESSOR OF UV-B PHOTOMORPHOGENESIS 1(RUP1)and RUP2 directly interact with monomeric UVR8 and facilitate UVR8 ground state reversion,completing the UVR8 photocycle.Here,we reconstituted the RUP-mediated UVR8 redimerization process in vitro and reported the structure of the RUP2-UVR8^(W285A) complex(2.0A).RUP2 and UVR8^(W285A) formed a heterodimer via two distinct interfaces,designated Interface 1 and 2.The previously characterized Interface 1 is found between the RUP2 WD40 domain and the UVR8 C27 subregion.The newly identified Interface 2 is formed through interactions between the RUP2 WD40 domain and the UVR8 core domain.Disruption of Interface 2 impairedUV-B induced photomorphogenic development in Arabidopsis thaliana.Further biochemical analysis indicated that both interfaces are important for RUP2-UVR8 interactions and RUP2-mediated facilitation of UVR8 redimerization.Our findings suggest that the two-interface-interaction mode is adopted by both RUP2 and COP1 when they interact with UVR8,marking a step forward in understanding the molecular basis that underpins the interactions between UVR8 and its photocycle regulators.